This invention relates to television, and more particularly to an improved method and apparatus for increasing the vertical resolution of color television using the method disclosed in copending application Ser. No. 06/515,220, now U.S. Pat. No. 4,589,012.
Briefly, vertical resolution is increased for standard television in accordance with the invention disclosed in the aforesaid patent application by modulating the vertical scan axis (deflection field) of a television camera such that each active scanning line traverses an undulatory path in phase from line to line rather than a straight line, i.e., oscillates in phase from line to line about the normal scan lines. The amplitude of the modulation is preferably sufficient for interlaced lines of a frame to intrude the area of adjacent lines. The video signal thus produced by actually scanning areas above and below the normal scan lines is then transmitted as for the normal scanning lines, which can be within the vestigial bandwidth limit of about 4 MHz for NTSC broadcast transmission, as described in U.S. patent application Ser. No. 06/534,592, now U.S. Pat. No. 4,620,217.
At a monitor or television receiver, a local oscillator modulates the vertical scan axis of each line to be at the same frequency and phase as the modulation employed in the television camera to synchronously reproduce each line of a frame. The local oscillator may be synchronized with the television camera by information contained in the received video signal. For example, in NTSC color transmission, a gated burst of the color carrier is transmitted during horizontal blanking periods for use in regenerating the color reference in the monitor or television receiver referred to generally as the display unit. Consequently, a convenient modulation frequency is the fundamental, i.e., the same frequency as the color carrier (3.579545 MHz under the NTSC system), or preferably some multiple or submultiple of that frequency. In any case, if the modulation in the television camera and the display unit are synchronized and in phase from line to line, the display unit will reproduce the frame in the same manner as it was scanned in the camera, which is with increased vertical resolution.
The second harmonic (7.15909 MHz), or some multiple of the second harmonic of the color carrier frequency is preferred over the fundamental (3.579545 MHz) because, at the standard 15.735 kHz line frequency, each line will then have a whole number of modulation cycles, such as 455 for the second harmonic, rather than a fractional number of modulations, such as 227.5 for the fundamental, thus making it easier to maintain the same modulation phase from line to line. However, a fraction of the color subcarrier frequency may also be used, such as one half, to produce modulation of a line with one whole number of cycles to facilitate the need to synchronize the phase of the line modulation from line to line.